But setting nullable, doesn't work for me, although it sounds like the correct way to do it. I will use one SPSiteDataQuery now for each of the two lists and merge the results (I'll also check SPQuery against it for performance issues).

Unfortunately that didn't work. Added Nullable for the CustomField to ViewFields and/or the Query doesn't work. Also firing a query with a non existent field via "Stramit CAML Viewer" throws an exception.
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HutzelGutzelJan 9 '12 at 15:09

That's really bad news. Although I haven't tried this code myself, several MSDN articles I've read provide straight examples of doing this. Can you show us the exception message and its stack trace? (you can add this information to your question)
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Andrey MarkeevJan 9 '12 at 16:55

The exception only occurs in the CAML Viewer tool and is not helpful. It only prints "WebserviceException". In the application there are no results returned when querying nonexistent fields.
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HutzelGutzelJan 10 '12 at 8:05